Substantial quantities of titanium dioxide pigment are produced commercially by reacting titanium tetrachloride with oxygen in the vapor phase. After such reaction, the titanium dioxide reaction mass is cooled by passing it through a conduit, i.e., flue, where growth of the titanium dioxide pigment particles takes place and agglomeration of such particles takes place.
It is desirable to cool the titanium dioxide rapidly because this will result in pigment having higher carbon black undertone ("CBU"). CBU is described in U.S. Pat. No. 2,488,440, which is hereby incorporated by reference.
The cooling of the titanium dioxide can take place in a conventional flue, i.e., a conduit having a substantially smooth interior. Or, if desired, a finned flue can be used, i.e., a conduit having a plurality of substantially longitudinal, internal, protuberances, depressions or both. Suitable finned flues are described in U.S. Pat. No. 4,937,064, which is hereby incorporated by reference.
However, as production rates increase or there is a need for additional cooling, the interior diameter of the flue sometimes needs to be increased so that it is larger than the interior diameter of the outlet of the reactor to oxidize TiCl.sub.4 to TiO.sub.2. The need therefore exists for a suitable transition piece.